Thermal and mechanical properties of poly(L-lactic acid)-poly (ethylene co-vinyl acetate) blends

Poly(L-lactic acid) (PLLA) was blended with poly(ethylene-co-vinyl acetate) (EVA) and the miscibility of the blend was investigated by DSC, polarizing optical microscopy and rheometry. The blend of PLLA with EVA70, that contained 70 wt% of vinyl acetate, was immiscible because the Tg and the spherulitic growth rate of the blend were nearly constant regardless of the change in the blend composition. On the other hand, the Tg, equilibrium melting temperature, the spherulitic growth rate during the isothermal crystallization of the PLLA-EVA85 blend were decreased with the increase in the EVA content. Moreover, the Flory-Huggins interaction parameter of the blend was found to be negative, which clearly means the PLLA-EVA85 blend was miscible. The tensile strength and modulus of the PLLA-EVA85 blend were dropped rapidly, followed by a more gradual decrease with the increase in the EVA85 content. Strain-at-break, however, was increased rather slowly up to 70 wt% of EVA85 and then increased quite rapidly around 90 wt% of EVA85.